A Review of the Nutritional Approach for Children with Autism Spectrum Disorders (PDF)

nutrients Review A Review of the Nutritional Approach and the Role of Dietary Components in Children with Autism Spectrum Disorders in Light of the Latest Scientific Research Seda Önal 1,2 , Monika Sachadyn-Król 3 and Małgorzata Kostecka 3, *...

nutrients Review A Review of the Nutritional Approach and the Role of Dietary Components in Children with Autism Spectrum Disorders in Light of the Latest Scientific Research Seda Önal 1,2 , Monika Sachadyn-Król 3 and Małgorzata Kostecka 3, * 1 Department of Nutrition and Dietetics, Health Sciences Institute, Ankara University, 06110 Ankara, Turkey; [email protected] 2 Department of Nutrition and Dietetics, Faculty of Health Sciences, Fırat University, 23200 Elazığ, Turkey 3 Faculty of Food Science and Biotechnology, University of Life Sciences in Lublin, 20-950 Lublin, Poland; [email protected] * Correspondence: [email protected] Abstract: Autism spectrum disorder (ASD) is a neurodevelopmental disorder that affects several areas of mental development. The onset of ASD occurs in the first few years of life, usually before the age of 3 years. Proper nutrition is important to ensure that an individual’s nutrient and energy requirements are met, and it can also have a moderating effect on the progression of the disorder. A systematic database search was conducted as a narrative review to determine whether nutrition and specific diets can potentially alter gastrointestinal symptoms and neurobehavioral disorders. Databases such as Science Direct, PubMed, Scopus, Web of Science (WoS), and Google Scholar were searched to find studies published between 2000 and September 2023 on the relationship between ASD, dietary approaches, and the role of dietary components. The review may indicate that despite extensive research into dietary interventions, there is a general lack of conclusive scientific data about the effect of therapeutic diets on ASD; therefore, no definitive recommendation can be made for any specific nutritional therapy as a standard treatment for ASD. An individualized dietary approach Citation: Önal, S.; Sachadyn-Król, M.; and the dietician’s role in the therapeutic team are very important elements of every therapy. Parents Kostecka, M. A Review of the and caregivers should work with nutrition specialists, such as registered dietitians or healthcare Nutritional Approach and the Role of providers, to design meal plans for autistic individuals, especially those who would like to implement Dietary Components in Children an elimination diet. with Autism Spectrum Disorders in Light of the Latest Scientific Research. Keywords: autism spectrum disorders; children; elimination diet; dietary component; eating disorders Nutrients 2023, 15, 4852. https:// doi.org/10.3390/nu15234852 Academic Editors: Manuela García de la Hera, Laura Torres-Collado 1. Introduction and Laura María Compañ Gabucio Autism spectrum disorder (ASD) is a multifaceted neurodevelopmental condition Received: 29 September 2023 that is defined by the presence of core symptoms such as restricted/repetitive behaviors, Revised: 15 November 2023 language delay, and social interaction/communication impairment. In addition to Accepted: 16 November 2023 these definitions, an ASD diagnosis often co-occurs with other conditions, including motor Published: 21 November 2023 abnormalities, gastrointestinal (GI) problems, epilepsy, intellectual disability, and sleep disorders. ASD typically manifests before the age of 3 years and may persist throughout an individual’s lifetime. According to the Centers for Disease Control and Prevention (CDC) report of March 2023, 1 in 36 children aged 8 years is diagnosed with ASD, Copyright: © 2023 by the authors. marking an increase from 1 in 44 children in the previous reporting period. ASD is nearly Licensee MDPI, Basel, Switzerland. four times more common among boys than girls. This article is an open access article The etiology of ASD is still poorly understood, and ASD is considered to be a mul- distributed under the terms and conditions of the Creative Commons tifactorial disorder caused by genetic, epigenetic, and environmental factors [1,5]. It is Attribution (CC BY) license (https:// emphasized that genetic factors are responsible for only 10–20% of ASD cases, and accord- creativecommons.org/licenses/by/ ing to the literature , nearly half of the symptoms associated with ASD may be due to 4.0/). Nutrients 2023, 15, 4852. https://doi.org/10.3390/nu15234852 https://www.mdpi.com/journal/nutrients Nutrients 2023, 15, 4852 2 of 22 environmental factors. In the group of environmental factors, nutrition plays a critical role in the development of ASD. The levels of glutathione (GSH), a primary intracellular antioxidant and detoxifying agent which acts as a methyl donor for many metabolic reactions, and S-adenosylmethionine (SAM) are significantly lower in ASD due to oxidative stress. Specific nutritional factors, including sulfur amino acids such as cysteine (CYS) and methionine (MET), as well as folate, vitamins B12, and B6, play a critical role in supporting the metabolic pathways responsible for GSH and SAM. These factors are of paramount significance in the dietary requirements of individuals with ASD. Mitochondria play a key role in oxidative stress and in maintaining the cellular redox state. Dietary supplements containing nearly all nutri- tional factors that support the metabolic pathways generating GSH and SAM have been shown to improve hepatic mitochondrial function in mice with diet-induced obesity [6–8]. Children with ASD experience more general GI symptoms, including abdominal pain, diarrhea, and constipation. It has been reported that 83–91% of ASD individuals have co-occurrence of GI symptoms. Children with ASD who are unable to express their emotions are known to manifest GI symptoms in extreme behaviors such as aggression, self- injury, and excessive self-repetitive behavior. Given the prevalence of GI symptoms in ASD and other key behavioral areas, new research has been undertaken to explore the changes in the gut microbiota and the metabolites that may be associated with these symptoms. In individuals with ASD, altered gut microbiota can influence the immune system and lead to the release of metabolites, which points to a connection between dysbiotic gut microbiota and ASD [13–15]. Due to the ambiguity of the causal relationship, the relative inefficacy of ASD treatment strategies, and insufficient knowledge about the role of dietary factors in the development of ASD, patients are currently being exposed to numerous “complementary and alternative interventions” by researchers and caregivers. Some of these interventions specifically3target Nutrients 2023, 15, x F R PEER REVIEW of 26 the diet and gut health [16–18]. Figure 1 summarizes the problems that may be associated with nutrition and nutritional therapies in ASD. Figure 1. Problems that may be related to nutrition in ASD and current nutritional therapy approaches. Figure 1. Problems that may be related to nutrition in ASD and current nutritional therapy ap- proaches. Do children with ASD eat differently? Do they have eating disorders that require dietary modification and separate dietary patterns? Can the described dietary modifications 2. Methods or elimination diets be applied to children with ASD and is such dietary management safe The review for them? was performed To answer by a team these questions, of Polish the main and Turkish objective of this researchers review was as to part of a describe broader study on the role of diet in alleviating ASD in Polish and Turkish children. The study involved a narrative literature review as a comprehensive, critical, and objective analysis of the current knowledge on the topic of ASD and dietary components. Electronic databases such as Science Direct, PubMed, Scopus, Web of Science, and Google Scholar were searched to find related studies published between 2000 and September 2023 (all Nutrients 2023, 15, 4852 3 of 22 the existing clinical and experimental knowledge and to determine whether nutrition and specific diets can potentially alter gastrointestinal symptoms and ASD. 2. Methods The review was performed by a team of Polish and Turkish researchers as part of a broader study on the role of diet in alleviating ASD in Polish and Turkish children. The study involved a narrative literature review as a comprehensive, critical, and objective analysis of the current knowledge on the topic of ASD and dietary components. Electronic databases such as Science Direct, PubMed, Scopus, Web of Science, and Google Scholar were searched to find related studies published between 2000 and September 2023 (all available articles were analyzed in the case of the Feingold diet). A comprehensive search was performed using the following keywords: autism, autistic disorder, ASD, autism spectrum conditions, cross-sectional study, nutrition, elimination diet, restriction diet, and nutritional approach. All related studies were identified and transferred into the EndNote software (version 20.6) to select and manage the references. The reference lists of the related studies were also examined manually to find other potentially eligible studies. A total of 155 articles were ultimately selected for review. 3. Selective Eating as a Nutritional Problem Selective eating is generally defined as restrictive eating behavior with a limited number of preferred foods. Selective eating is often associated with eating disorders, in particular when it has negative health or psychosocial consequences. There is no con- sistent operational definition of selective eating, which is why the prevalence of selective eating is difficult to estimate in children both with and without ASD. According to current estimates, selective eating is more prevalent in children with ASD than in typically develop- ing children, where it has been estimated at 15–20%. A literature review conducted by Ledford and Gast (2006) demonstrated that behavioral feeding disorders affect 46–89% of children with ASD. In the reviewed studies, selective eating was generally associated with frequent food refusals, a limited range of foods, high intake of a few acceptable foods, and selective preferences for some groups of food products. Children with ASD have not developed healthy eating skills, which is why they face challenges during mealtimes. In individuals with ASD, food preferences, aversions, and food refusal could be influenced by sensory sensitivity to textures, tastes, and smells. Individuals with ASD often experience GI problems such as abdominal pain, constipation, and diarrhea, which could be related to selective eating. Oral motor impairments and fine motor impairments in persons with ASD can influence swallowing, chewing, and utensil use, which further contributes to feeding difficulties. Behavioral inflexibility and a need for sameness are some of the core challenges in ASD, and they prevent diverse eating experiences which are characterized by frequent changes in menu, utensils, dishes, and environments. Mealtime is also a social activity that involves interaction and conversation. In ASD, the high prevalence of co-occurring anxiety increases the risk of food neophobia, namely the fear of trying new foods. Children have to be repeatedly exposed to novel and potentially anxiety-inducing foods to develop healthy food preferences and eat a varied diet. Individuals with ASD often lack the cognitive skills for managing novelty, forming prototypes, and generalization, and they could find it difficult to process the similarities and differences in food groups, including differences in the color, flavor, and consistency of cheese. In the work of Yamane et al. (2020), children with food selectivity were classified based on the type of encountered difficulties (Table 1). Nutrients 2023, 15, 4852 4 of 22 Table 1. The type and the developmental characteristics of autistic children with selective eating habits according to the classification proposed by Yamane. Tendency Brief Characteristics Foods are selected based on Severe intellectual disability (the sensory factors, such as sensory motor stage in the theory of Group 1 (sensory) texture, smell, taste, mental development) temperature, and color Oral hypersensitivity Foods are judged at a glance Moderate intellectual disability Group 2 (visual) based on color, shape, or Severely restricted imagination cooking method Visual performance is dominant Developmental age for cognition Group 3 (familiarity) Selection of familiar foods >2 years Difficulty in social imagination (prediction) Food choices are affected by Attention deficit disorder Group 4 (environmental environmental factors: place, Each developmental age stimulation) plate, cup, people, and room is variable temperature In a recent study by Amin et al. (2022), individuals with ASD consumed 44 out of the 75 food items listed in the Food Preference Questionnaire (FPQ), and they were more selective than typically developing (TD) participants who ate 51 food items on average. Similar observations were made in previous studies. Children with ASD consumed a smaller variety of foods than children without (22.8% versus 3.5%, p = 0.002). In the studied population, 35% of children with ASD, but only 3.5% of the control subjects, exhibited food selectivity toward starchy foods. Children with ASD most often refused to eat meat, eggs, rice, vegetables, and fruits. In the group of 279 patients who were assessed over a period of 24 months, 70 children with ASD and severe food selectivity were eligible for inclusion in the study. According to the surveyed caregivers, vegetables were excluded by 67% and fruits were excluded by 27% of the studied population. In 78% of the children, the consumed diets increased the risk of five or more nutritional deficiencies. Malhi et al. (2017) also found that children with ASD ate fewer foods, in particular fruits, vegetables, and proteins, than typically developing children. Children with ASD refused more food items and were six times more likely to be picky eaters than the control subjects. The foods offered at home were also analyzed to rule out the influence of the family’s dietary choices on the food intake of children with ASD. Interestingly, no significant differences were reported in the number of consumed foods from each food group, or the total number of foods consumed by families with and without autistic children. In addition, the diets consumed by the parents of children with ASD were significantly more varied than the children’s diets, which clearly indicates that food selectivity in children with ASD did not result from food restrictions in the family. Postorino et al. (2015) evaluated the clinical and behavioral characteristics of autistic children aged 3–11 years and compared children with and without eating problems, including GI symptoms, food refusal, and food selectivity. The severity of ASD symptoms was assessed by both professionals (ADOS- G) and parents (Autism Diagnostic Interview-Revised—ADI-R, Social Responsiveness Scale—SRS, Social Communication Questionnaire—SCQ). Interestingly, the prevalence of ASD symptoms in children with food selectivity was significantly higher in parental assessments than in professional assessments. Similar findings were reported by Allen et al. , Zachor and Ben-Itzchak , and Prosperi. According to Saban-Bezalel et al. (2021), this discrepancy could be attributed to the continuous impact of atypical eating habits throughout childhood, which can influence parental perceptions of symptom severity. In turn, professionals are generally not familiar with the long-term eating habits of the assessed children. Nutrients 2023, 15, 4852 5 of 22 Restrictive food intake in a selective diet can lead to nutritional deficiencies and pose a potential health risk associated with feeding problems. However, research investigating nutrient intake in autistic children produced contradictory findings. According to a meta- analysis of 56 studies conducted by Bourne et al. (2022), low body weight or substantial weight loss are the most noticeable outcomes of a severely limited diet, especially in children where a restrictive diet may undermine the achievement of the expected growth and developmental milestones. Despite the above, Avoidant/Restrictive Food Intake Disorder (ARFID) is not always correlated with low weight. In some cases, autistic children and adolescents were overweight due to excessive consumption of a narrow range of energy- dense foods high in fat, sugar, or salt. Low dietary variety can have significant consequences, and it can deprive individuals of essential nutrients such as iron and vitamins [37,38]. The results of recent studies investigating the nutrient intake of children with ASD are presented in Table 2. A meta-analysis of 17 prospective controlled trials revealed that protein and calcium intake was considerably lower in children with ASD than in typically developing children. Several researchers also reported a higher prevalence of iron deficiency and anemia among children with ASD. In other studies, healthy children were characterized by significantly higher average levels of hemoglobin, ferritin, magnesium, potassium, calcium, phosphorus, glucose, and hematocrit than autistic children. Table 2. Nutrient intake in children with ASD. Nutrient Intake Subjects Source Protein Lower in ASD Other Macronutrients, No difference Vitamin A Lower in ASD 86 children with Vitamin B1, B2, B3, B6 Lower in ASD ASD aged Folic Acid Lower in ASD 2–8 years and Barnhill et al. Vitamin B12 Lower in ASD 57 age-matched Calcium Lower in ASD peers without Iron Lower in ASD ASD Zinc Lower in ASD Selenium Higher in ASD 63 ASD children in the age range of Energy No difference 4 to 10 years and Fats No difference 50 typically Potassium Lower in ASD developing Malhi et al. Copper Lower in ASD children matched Folate Lower in ASD on age and Iron Lower in ASD socio-economic Vitamin C Lower in ASD status to the ASD children Protein Adequate (RDA) 53 children with Calcium Lower than RDA ASD (45 boys and Siddiqi et al. Iron Lower than RDA 8 girls) in the age Zinc Lower than RDA group of Vitamin B2 Lower than RDA 2–13 years Fiber Lower than DRI Calcium Lower than DRI Vitamin E Lower than DRI 70 children with Sharp et al. Vitamin A Lower than DRI ASD and severe Vitamin C Lower than DRI food selectivity Folic Acid Lower than DRI Zinc Lower than DRI Nutrients 2023, 15, 4852 6 of 22 Table 2. Cont. Nutrient Intake Subjects Source 738 ASD children Folate Lower in ASD and 302 typically Zinc Lower in ASD Zhu et al. developing Vitamin B12 Lower in ASD children (TD) Vitamin D Lower in ASD age 2–6 Vitamin E Lower in ASD Vitamin K Lower in ASD Vitamin B2 Lower in ASD Vitamin B6 Lower in ASD 52 ASD cases Vitamin A Lower than DRI in ASD & TD (37 boys and Vitamin D Lower than DRI in ASD & TD 15 girls), 51 TD Alkhalidy Vitamin B12 Lower than DRI in ASD &TD children (26 boys et al. Folate Lower than DRI in ASD & TD and 25 girls) Magnesium Lower than DRI in ASD & TD aged 3–6 Phosphorus Lower than DRI in ASD & TD Zinc Lower than DRI in ASD & TD Selenium Lower than DRI in ASD &TD RDA—Recommended Dietary Allowance; DRI—Dietary Reference Intake. 4. Nutritional Approaches 4.1. Diets 4.1.1. Gluten-Free and Casein-Free Diets High levels of urinary peptides have been identified in children with ASD [45–47]. Many of these peptides can be classified as exorphins (exogenous opioids), including casomorphins and gluteomorphin derived from dietary sources containing gluten and casein. This obser- vation provides further evidence for incomplete protein digestion and increased intestinal permeability in individuals with ASD [48,49]. Circulating peptides can cross the blood–brain barrier, directly influence the central nervous system, and exert adverse effects on attention, brain development, social communication, and learning. [50,51]. It has been hypothesized that a diet low in these proteins may normalize urinary peptide levels and improve the behavioral symptoms of affected children. Several re- cent studies analyzing the effects of dietary treatments on ASD [52–57] are presented in Table S1 in Supplementary Materials. In most studies examining the impact of a GFCF diet on autistic children, the studied population is usually small, and the diet is applied for a short period of time. Prolonged adherence to a GFCF diet could lead to nutrient deficiencies, social isolation of children with ASD (similarly to other restrictive diets), and considerable economic burden for families. Therefore, a GFCF diet is not fully beneficial for children with ASD. 4.1.2. Ketogenic Diet The ketogenic diet (KD) is high in fat, extremely low in carbohydrates, and low in proteins. Individuals who adhere to the KD obtain 90% of dietary energy from fat, 7% from protein, and 3% from carbohydrates. In individuals with ASD, KD may improve social behavior by normalizing GABA, enhancing mitochondrial function, reducing inflammation and oxidative stress in the brain, inhibiting the mTOR signaling pathway, and modulating the gut microbiota. Researchers analyzing the effects of the KD did not observe significant behavioral improvements in animal models of ASD, and preliminary human studies demonstrated that KD was effective in improving the core symptoms of ASD [59,60] (Table 3). The efficiency of the KD must be monitored with the use of urinary ketones and serum beta-hydroxybutyrate (BHB). Nutrients 2023, 15, 4852 7 of 22 Table 3. The ketogenic diet as potential treatment for ASD. Study Group Scientific Evidence References Animal Studies The study analyzed the protective The study reported on a KD-related partial effects of a KD on sociability, spatial restoration of social features and alleviation learning, memory, and of seizure events in male subjects. The KD electroencephalogram seizures in also exerted neuroprotective effects in Dai et al. glut3 heterozygous null (glut3+/−) female subjects due to higher circulating and mice exhibiting features relevant cerebrospinal fluid ketone concentrations to ASD. and/or lower brain Glut3 concentrations. A clear sex-related difference was reported The study examined mutant EL in response to the KD. The KD improved mice with comorbid epilepsy and multiple measures of sociability and Ruskin et al. ASD symptoms. reduced repetitive behavior in female mice, but had limited effects in males. The early timing of a dietary intervention was recognized as an important factor in diet-dependent brain reorganization and The experiments involved En2 maturation. Although monoamine levels in knockout mice exposed to a KD Verpeut et al. the forebrain regions were not affected in from postnatal day 21 to 60. two null mice (En2(−/−)), increased social contact and reduced grooming behavior were evident in response to KD intervention. Male MIA offspring were significantly asocial in the three-chamber sociability test, while female mice displayed normal and The study analyzed sociability and social behavior. After 3–4 weeks of KD social behavior in male and female Ruskin et al. treatment, the lack of sociability in male MIA mice offspring was completely reversed and MIA-induced, self-directed, repetitive behavior was reduced. Human studies A KD reduced autistic manifestations in the Autism Treatment Evaluation Test (ATEC) 45 children aged 3–8 years and the Childhood Autism Rating Scale El-Rashidy et al. (CARS), in particular by improving sociability. A modified ketogenic gluten-free diet supplemented using medium-chain triglycerides (MCTs) improved the social 15 children aged 2–17 years affect subdomain and scores in the total Lee et al. autism diagnostic observation schedule, 2nd edition (ADOS-2) scores, but it had no effect on restricted and repetitive behavior scores. A KD improved social communication in one of the six ASD patients and reduced the Six ASD (aged 4–14 years old) prevalence of comorbidities in patients, patients with a pathological increase including attention deficit hyperactivity Spilioti et al. in beta-hydroxybutyrate disorder (ADHD), compulsive behavior, preoccupation with parts of objects, and abnormal sleep. In a case study of a child with ASD, a KD improved behavior and intellect, and 6-year-old child Żarnowska et al. decreased the 18F-FDG uptake in the whole cortex. It should be noted that despite encouraging results, human studies examining the applicability of the KD in subjects with ASD have several important limitations. These limitations include smaller sample sizes, difficulty in adhering to the KD, discrepancies between the duration and composition of the KD, high dropout rates resulting from the unpalatability of the KD, and nutritional deficits. In addition, the KD is associated with a higher risk of inflammation and mitochondrial dysfunction, as well as adverse effects such as constipation, reflux, and other comorbidities. The side-effects of the KD described in the literature are presented in Table 4. The KD seems effective in ASD patients, but the reviewed clinical studies had small sample sizes, probably because randomized trials are Nutrients 2023, 15, 4852 8 of 22 difficult to set up for children with ASD. Additional studies are warranted to understand the effects of the KD in individuals with ASD. Table 4. Why a ketogenic diet is not the best diet for ASD. Most Common Side Effects References The KD can be difficult to maintain, especially in children with limited food preferences. It is important to have a plan in place to ensure that the child is able to stick to the diet. ASD patients also consume fewer foods and exhibit more feeding problems and diverse eating behaviors (selective intake, food refusal, food aversion, and Li et al. atypical eating). Some foods are refused due to presentation or the Mayes & Zickgraf need to use certain utensils. In a study by Albers et al. 73% of the respondents rated adherence to the KD as more difficult, compared with age-matched controls, whereas only 26% of the subjects did not report such difficulties. These results confirm that the administration of a KD to ASD children is difficult. The sensory abnormalities commonly associated with ASD can influence the administration of and adherence to the KD. Parents reported that children with ASD were significantly more averse to Albers et al. food textures (p < 0.0001), in particular foods with a slimy and creamy Balasco et al. texture. According to the authors, taste preferences and consistent Cermak et al. food routines are important or very important determinants of the successful implementation of the KD. Taste, smell, and texture hypersensitivities/aversions were regarded Albers et al. as the key difficulties in the implementation of a KD Nutrient Deficiencies: The KD is very restrictive, and it may not provide growing children with the necessary nutrients. In children, the KD may suppress physical development and cause height Spulber et al. deceleration. Parents should work with healthcare providers or dietitians to eliminate the risk of nutritional deficits. Possible Side-Effects: Some children may experience side effects from the KD, such as constipation, nausea, and vomiting. During the initial Neal et al. phase of the diet, common side effects also include hypoglycemia, Newmaster et al. metabolic acidosis, and refusal to eat. 4.1.3. Feingold Diet In addition to the most common elimination diets, various dietary modifications have been sought to treat children with ASD. One such modification was polarized in the 1970s by the controversial Feingold diet. This dietary regime involves the elimination of artificial food dyes, flavors, sweeteners, preservatives, as well as foods containing butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tert-butylhydroquinone (TBHQ), and salicylates. Despite considerable success, the Feingold diet has been widely criticized by the medical community for a lack of scientific evidence, strict rules, and potential health risks. In later years, several controlled studies were conducted to compare the behavior of hyperactive children on the Feingold diet and a placebo diet (diet crossover designs), or to examine specific behavioral responses to the presence of food dyes in the diet. In one study, autistic children aged 3–6 years old were placed on the Feingold diet coupled with a language training program. Non-significant differences in language education performance and the development of receptive language skills were observed before and after the intervention. Several reviews [80–82] and one meta-analysis have concluded that the Feingold diet is not an effective treatment for hyperactivity. Schab and Trinh (2004) conducted a meta-analysis focusing on the effect of artificial food dyes on hyperactivity rather than the Feingold diet as a whole. Their results corroborate the hypothesis that artificial food dyes can increase hyperactivity in some children. On the other hand, reduced phenol sulfotransferase (PST) activity in ASD individuals compared to controls points to the feasibility of the Feingold diet. Nutrients 2023, 15, 4852 9 of 22 4.1.4. Candida Diet The influence of Candida spp. on the immune system, brain, and behavior of children with ASD has been examined by many researchers. Numerous studies demonstrated that Candida spp. were isolated more frequently from the stool samples of autistic patients than healthy subjects. Doreswamy et al. (2020) found much higher yeast counts in the autistic group than the control group. Iovene et al. (2017) also identified aggressive forms (pseudohyphae) of Candida spp. in stool samples from 57% of children with ASD In the work of Emam et al. , the Candida spp. counts were also much higher in individuals with ASD (81.9%) than in healthy controls (28%), but the observed differences were not statistically significant [89,90]. In some studies, yeasts were often identified under a microscope, but they were rarely observed in samples collected from autistic or control groups, which points to the high ambiguity of the reported results. The following Candida species were most frequently isolated from the stool samples of children with diagnosed or suspected ASD: C. albicans (57.4%), fluconazole-resistant C. krusei (19.8%), and C. glabrata (14.8%) [92,93]. None of these pathogens were identified in healthy children. Most studies demonstrated that increased counts of Candida spp. did not affect the severity of symptoms in autistic children and were not correlated with GI symptoms [87,92]. Research conducted in the 1980s suggested that yeasts play a significant role in behavioral and learning problems in children with ASD. It was noted that excessive and long-term exposure to antibiotics (for example, in the treatment of middle ear infection) led to gut dysbiosis and caused the overgrowth of C. albicans yeast in the intestinal tract. Srikantha and Mohajeri (2019) reported that Candida spp. may increase serotonin production in the intestine (5-hydroxytryptamine, 5-HT) and reduce serotonin synthesis in the brain (due to the consumption tryptophan, a precursor of serotonin), which can lead to hyperserotonemia and behavioral outcomes in ASD children. Autistic children are picky eaters, and their diet usually consists of a very narrow range of foods, depending on type, texture, or appearance. These children have a preference for starchy and fatty foods, simple carbohydrates, snacks, and processed foods. Diets abundant in carbohydrates, such as glucose and mannose , correlated positively with the abundance of gut Candida , whereas negative correlations were observed between high-protein diets and the abundance of fungi in healthy volunteers. Furthermore, mannitol, sorbitol, xylose, adonitol, and xylitol were also found to significantly promote the growth of C. albicans (100–200%), whereas other metabolites, such as raffinose, arabinose, trehalose, lactose, galactinol, galactitol, and arabitol, had a low or marginal impact on the proliferation of C. albicans. Added sugars (including honey, jam, and candy), highly refined carbohydrates (in particular, flour-based products), red and cured meats, and dairy products should be avoided to prevent the growth of Candida spp.. Anti-Candida diets have been recommended for autistic children due to their health benefits ; however, there is no conclusive empirical evidence to indicate that these diets inhibit Candida growth or promote gut health. The information about the efficacy of these diets is insufficient to make recommendations for their use. Probiotics appear to be a promising treatment for reducing GI disturbances and the overgrowth of Candida in children with ASD [103,104]; however, the efficacy of probi- otics remains controversial and requires further research. Probiotics such as Sacharomyces boulardii and Lactobacillus spp. may induce changes in the intestinal microflora and inhibit the growth of pathogens by stimulating the production of β-defensin and IgA. The proliferation of Candida can be suppressed by IL-17 and IL-22, which are modulated by selected Lactobacillus species via tryptophan-derived aryl hydrocarbon receptor lig- ands. Probiotics may strengthen the intestinal barrier by maintaining tight junctions and triggering mucin production. The composition of the gut microbiota could be modulated to minimize Candida overgrowth, GI problems, and ASD symptoms; however, human studies have provided limited or inconclusive evidence that a sugar-free diet or probiotic supplementation deliver Nutrients 2023, 15, 4852 10 of 22 beneficial effects in ASD. The current findings do not promote the use of these modalities in the treatment of ASD or the introduction of such changes into the patients’ diets. 4.1.5. Specific Carbohydrate Diet This diet (SCD) was developed by Dr. Sidney Haas for the treatment of celiac disease. This diet was developed on the assumption that carbohydrates (sugars) feed bacteria and yeasts in the intestines, thus leading to an over-abundance of bacteria and yeast. As a result, carbohydrates remain undigested in the intestines, which provides even more food for bacterial and yeast growth. Many autistic children have severe GI symptoms, including diarrhea, constipation, bloating, and pain. The associated functional GI abnormalities include low levels of disaccharidase enzymes [108,109] and defective sulfation of ingested phenolic amines, such as acetaminophen. Some ASD specialists believe these symptoms could be caused by bacterial or fungal overgrowth in the intestines [91,111]. The SCD eliminates complex starches that feed bacteria and yeasts in the intestines, which improves ASD symptoms and mitigates functional GI abnormalities. In the SCD, foods are described as “legal” or “illegal” based on their carbohydrate content. “Illegal” carbohydrates include all cereals, (maize, wheat, wheat germ, barley, oats, and rice), sugars, beans, potatoes, and all processed foods (including canned and preserved vegetables). Milk and milk products with a high lactose content, ice cream, sweets, chocolate, and products containing fructooligosaccharides (FOS) are also placed on the list of prohibited items. Seaweed and related products should also be eliminated from the diet. “Legal” carbohydrates include unprocessed meats, vegetables, fruits, and some dairy products (a casein-free version of this diet has also been proposed). Certain legumes, including dried navy beans, lentils, peas, split peas, unroasted cashews, shelled peanuts, all-natural peanut butter, and lima beans, are also allowed. Most oils, teas, coffee, mustard, apple or white vinegar, and juices without additives or sugars can be a component of a varied diet, and honey can be used as a sweetener in special cases [112,113]. The SCD is already naturally gluten-free. In a case study conducted by Barnhill et al. (2020), the SCD led to a considerable increase in protein intake which exceeded the current RDA levels. However, high-protein diets have been found to increase the intestinal absorption of calcium, increase the levels of circulating insulin-like growth factor-1, and decrease the serum parathyroid hormone level. In a recent study, the intake of higher animal protein, calcium, and phosphorus was positively associated with bone density measures in autistic children, and the authors concluded that children with ASD should focus on higher protein intakes than the RDA. Despite the fact that the effectiveness and safety of the SCD protocol for autistic children with GI problems has not been evaluated to date, this diet is widely used by many families, with or without clinical guidance. Restrictive dietary interventions may lead to nutritional deficiencies, especially when not clinically supervised or when the patients exhibit selective eating patterns and restricted dietary diversity. These observations indicate that the use of the SCD protocol in patients with ASD warrants additional investigation. 4.2. Supplements In the absence of curative treatments for ASD, and due to changes in the patients’ eating behavior, nutritional supplements and alternative medicine solutions have been extensively promoted among the families of individuals with ASD. Despite a relatively large number of studies on this subject, many myths and uncertainties still persist. The use of nutritional supplements, including omega-3 fatty acids and multivitamins, lacks support from current scientific evidence and should not be recommended as the official guidelines. Nutrients 2023, 15, 4852 11 of 22 4.2.1. Vitamins and Minerals Multivitamin and mineral supplements: Some findings suggest that multivitamin and mineral supplements may improve sleep and resolve digestive issues in autistic children. Additionally, such supplements can provide essential nutrients that may be lacking or insufficient in their diets (see Table 2). However, before starting any supplementation, the patients should consult a doctor or a dietitian to ensure that the supplements are safe and appropriate. The evidence available for most vitamin and mineral supplements is insufficient to endorse a therapeutic supplementation approach to ASD. The latest evidence on the impact of supplements on ASD symptoms and health parameters is presented in Table 5. The scientific evidence for certain dietary components is outdated and often controversial. For instance, there is a lack of recent experiments concerning vitamin C. The application of vitamin C in addressing ASD has not gained widespread traction as a therapeutic approach. An initial study assessing the impact of a moderate dose of multivitamin and mineral supplements on children with ASD concluded that autistic children are deficient in vitamin C and that supplementation leads to clinical improvement. However, the existing research has numerous limitations. A considerable portion of supplementation research is currently in the animal testing phase. For example, some studies revealed that selenium supplementation increased selenium levels, led to substantial improvement in social interactions and cognitive function, decreased repetitive stereotypical behaviors, and altered neurotransmitter levels in BTBR mice, which are the most widely used animal model for ASD research. The authors concluded that selenium exerts a potentially protective effect on the hippocampus of BTBR mice by regulating the neurotransmitter levels, reducing oxidative stress, and mitigating neuroinflammatory responses and neural cell injury. The impact of supplementation on pregnant women and the risk of ASD has been studied in recent years. Insufficient nutrient intake during pregnancy is linked with various adverse outcomes, including elevated risk of atypical behavior and neuropsychiatric conditions such as depression, anxiety, schizophrenia, ASD, and ADHD, as well as impaired cognition, visual problems, and motor deficits. The use of iodine supplementation during pregnancy, particularly when initiated prior to conception or during the first trimester, was shown to be more effective in preventing neurological damage. Table 5. Recent research studies investigating the impact of dietary supplementation on ASD symptoms. Supplement Intervention Subject Main Results References Mg: 50 mg for children aged The improvement in the overall score 2–3 years, 100 mg for children of the treated group was statistically aged 4–8 years, 200 mg for significant relative to the placebo children aged 9–12 years. group and the intervention group. Khan et al.. Vitamin B6: 25 mg for children The improvement in the cognition A randomized, Mg+B6 70 children with ASD aged 2–3 years, 50 mg for children and emotion score was statistically double-blind, placebo aged 4–8 years and 100 mg for significant. The improvement in the controlled study children aged 9–12 years. social, communication and sensory Duration of intervention: deficiency score was not 3 months statistically significant. An improvement in iron levels and a reduction in the overall severity score on the Sleep Clinical Global Impression Scale were observed. Reynolds et al.. 3 mg/kg/day of liquid ferrous Fe 20 children with ASD Actigraphy measurements did not A randomized sulfate for 3 months reveal significant improvements in placebo-controlled trial the primary outcome measure, i.e., sleep onset latency and wake time after sleep onset. Nutrients 2023, 15, 4852 12 of 22 Table 5. Cont. Supplement Intervention Subject Main Results References In most children (84.2%) exhibiting ASD, symptoms of restless legs, and Infusion of ferrous serum ferritin levels below 30 µg/L carboxymaltose (FCM) at 19 children with ASD experienced clinical amelioration DelRosso et al.. Fe 15 mg/kg up to a maximum dose (age: 4–11 years) and notably enhanced serum iron Retrospective study of 750 mg parameters following a sole intravenous ferric carboxymaltose (FCM) infusion. Intervention group 1 received powdered selenium supplement at 1 × 20 g/day; intervention Supplementation did not induce Triana et al. 65 children with ASD Se group 2 received a functional significant differences in total Randomized (age: 2–6 years) food product with a high content glutathione peroxidase levels. controlled trial of selenium (bovine heart extract) at 50 g/day Zn supplementation markedly A dietary nutraceutical formula reduced CARS scores in children containing Zn was administered 30 children with ASD with ASD. Zn for 12 weeks. The daily Zn dose Meguid et al. (age: 3–8 years) Serum Zn and metallothionein levels was adjusted to the participants’ increased significantly after Zn body weight in kg plus 15–20 mg. supplementation. Vitamin A induced changes in the Participants with low plasma composition of gut microbiota and retinol levels (